Imaging sensors generally use one or more arrays of detectors, such as charge-coupled devices (CCDs), to collect multiple input signals representative of the sensed image which are then output to a storage or display device. Ideally, if a fixed (flat) input signal is presented to an imaging sensor, an identical output value will be obtained for all detectors in the sensor. This is rarely the case due to variations in detector sensitivity, electronics paths and other factors. These variations, if viewed prior to correction, result in visible streaking and banding in images. Therefore, imaging sensors need to be equalized for use during normal operations so that these effects can be removed from the images before viewing.
Equalization of an imaging sensor generally includes applying processing to the detector outputs to correct for variations in the detector responses to uniform input signals. Such equalization is a necessary part of the image formation processing.
For space-based and airborne systems, equalization is affected by making special imaging collections of large flat areas of the earth, with as little variation in the scene as possible. Corrections are calculated based on these flat scenes, and then applied to all images taken with the sensor. Specialized hardware is sometimes used which allows sunlight to directly illuminate the imaging sensor in a manner that causes uniform illumination across the whole imaging array. Images resulting from these processes can then be used to derive equalization terms needed to remove streaks and bands from images collected by the system. Often both methods must be used together to derive an equalization of acceptable quality.
Current methods require either specialized image collections be made, or specialized hardware be added to the system in order to perform equalization. This is expensive and wasteful in that specialized hardware adds cost to the system, and special imaging collections require the use of image collection time that is then not available for other applications such as collection of saleable images. Therefore, it would be useful and economical to have a method for equalizing imaging systems that requires neither specialized hardware nor specialized image collections.